This invention pertains to an angle measuring device, and more particularly, to such a device characterized by a high degree of accuracy.
In the prior art, angle measuring devices, for example theodolites, have used primarily mechanical structure in order to measure angles. Some prior art apparatus have been constructed using both optical and magnetic records for generating a periodic signal or light pattern, the phase of which is used to compute an angle. Such prior art apparatus use frequency mixing to produce a phase signal indicating major divisions of mechanical rotation and for further resolution, produce a second phase signal further dividing the major divisions into a fixed number of smaller divisions. Such phase measurement methods do not provide apparatus which are as accurate as apparatus constructed in accordance with the instant invention.
Another problem affecting angle measuring apparatus in the prior art is that even in the most accurate of such instruments, the tolerances for machining rotating structure are insufficient to ensure angle measurement accuracy to within one second arc.
Yet another problem in prior art angle measuring apparatus, especially theodolites, relates to structure of the apparatus which provides a gravity-based reference for angle measurement. In the past, various types of pendulums and mercury arrangements have been used, such arrangements being very easily oscillated and, therefore, difficult to use. Damping systems provided in the past have limited the range over which the gravity-based reference can be used.
A general object of the present invention is to provide apparatus for measuring angles having a greater accuracy than such prior art apparatus by overcoming the above-mentioned problems.
According to a preferred embodiment of the invention, the proposed angle-measuring apparatus includes a circular disc having 21,600 equally-spaced magnetic marks encoded about the disc and concentric therewith. The disc is fixedly attached to a shaft that is rotated by an electric motor beneath the disc, such shaft extending above the top of the disc. Mounted on the shaft and extending at right angles therefrom over the disc are two sensors, each sensor including a photocell and light source aimed one toward the other. Two fixed posts are positioned opposite each other along an imaginary line drawn through the center of the disc, each post interrupting the light beam from one of the sensors each time it passes thereby. Two rotatable posts are positioned one opposite the other along an axis of rotation centered over, but independent of the shaft, each such rotatable post interrupting the light path of the other sensor each time it passes thereby. The angle defined for measurement is selected by positioning the rotatable posts so that the angle to be measured appears generally as an arc between one fixed post and one rotatable post with respect to the disc center. Since the rotatable posts are maintained substantially 180.degree. apart, a pair of vertical angles is formed, the same angle appearing generally between the other fixed post and rotatable post.
A reference signal is generated each time a sensor interacts with each of its associated posts. A magnetic pickup is used to detect each mark on the disc. Control circuitry is provided to count the marks detected between selected reference signals. By counting the marks included within each opposing vertical angle and averaging those counts, errors in only one of such counts which are caused by small off-axis variations in the rotatable structures are eliminated. Also included within the control circuitry is resolving circuitry, such circuitry accurately generating a periodic waveform between each sensed mark in order to resolve further the measurement of the defined angle into seconds of arc.
In another embodiment of the invention, the disc is oriented in a vertical plane and, in lieu of the fixed posts, a tube in the form of an endless loop which is partially filled with fluid is provided for interaction with one of the sensors. A portion of the tube is restricted to provide damping of fluid movement within the tube. A gravity-formed meniscus occurs at each fluid surface. The sensor senses the meniscuses on both sides and in this manner, along with a pair of rotatable posts, another sensor and control circuitry as above described, vertical angles having a gravity-formed datum are measured.
Thus, a more specific object of the invention is to provide angle-measuring apparatus having improved accuracy by measuring the angle in such a manner as to average out errors caused by off-axis machining of rotatable structure in the apparatus.
Another specific object of the invention is to provide such increased accuracy by generating a periodic signal between major divisions of arc and counting the periods thereof to resolve further a measured angle.
Yet another more specific object of the invention is to provide increased accuracy in the measurement of vertical angles with respect to a gravity-formed datum.